This invention relates generally to electronic ballasts for fluorescent lamps, and more particularly to electronic ballasts for fluorescent lamps, which operate in the high frequency range and which provide both for both dimmable and non-dimmable lighting operation.
Various types of electronic ballasts are known. Magnetic ballasts operate at the frequency of the input power sources, e.g., at 50 or 60 Hz. In recent years, it has been recognized that a considerable increase in efficiency, i.e., the amount of power expended in relation to the lumen output of fluorescent lights, can be obtained, in the order of 40%, if the lamps are operated at higher frequencies, e.g., at frequencies between 30 and 50 KHz.
These electronic ballasts comprise an input circuit with surge protection, a thermal switch for protection against equipment failures or short circuits, and a noise filter circuit; a full wave rectifier, usually a diode bridge with a smoothing filter across its output; an inverter with an amplifier and a second DC power supply; and an output transformer which is connected to the fluorescent lamps.
The most common fluorescent lamp ballasts are non-dimmable. They have no manual control of the amount of light emitted by the fluorescent lamps. But recently, dimmable types of high frequency ballasts have been disclosed which use a voltage to frequency converter, an amplifier, and a second DC power supply in the inverter, and apply power to the lamps over a frequency range. One such electronic dimmable ballast is disclosed in U.S. Pat. No. 5,192,897 (Vossough et al.). The lamp brightness varies in proportion to the frequency of the power applied to the lamps.
A problem with many of the existing ballasts is that the secondary of the output transformer, which is connected to the lamps is not fully isolated from the primary of the transformer, allowing for a feed back path to the AC power source. This creates a hazardous condition when lamps are being changed because the exposed socket can carry 50 or 60 Hz power. Whereas high frequency power (in the order of 30 to 50 KHz) can cause a burn if the skin is exposed to it, 50 to 60 Hz power can cause shock or electrocution.
In addition, conventional fluorescent ballasts operate one fluorescent lamp, or possibly two fluorescent lamps, but no more than that. Thus, the present invention, as will be discussed later, provides the means for operating at least two, if not more, fluorescent lamps from a single ballast.
The following U.S. patents are related to discharge lights or fluorescent lighting ballasts, such as: U.S. Pat. Nos. 5,363,020 (Chen et al.); U.S. Pat. No. 4,698,554 (Stupp et al.); U.S. Pat. No. 5,233,273 (Waki et al.); U.S. Pat. No. 5,334,915 (Ohsaki et al.); U.S. Pat. No. 5,323,090 (Lestician). However, none of these patents supply a single variable frequency control circuit to control the light intensity of one or more ballasts with each ballast capable of operating two to four lamps in a single lighting fixture. In addition, all of the ballast circuits shown in these patents have some form of "on-board" oscillator with feedback control. This feedback control is required for proper operation of their respective ballasts and, without it, would fail to operate. Furthermore, the '554 patent, the '273 patent and the '915 patent have the lamp terminals connected to the 120/277 volt (60 Hz) power supply, thereby creating a shock hazard. In contrast, as will be discussed below, the present invention does not use any type of feedback which makes the present invention a simple, reliable device which uses less than 1/2 the components used in the above-cited patents. Furthermore, the present invention also provides electrical isolation from the 50/60 Hz power system, which also makes the present invention inherently safe from shock hazard.
The reason a feedback circuit is added to the base frequency power system in a conventional ballast, or those cited above, is to automatically make an adjustment or change in the voltage, current or frequency of the power which is being used to light the fluorescent lamps or to correct a short coming in the basic design in one or more circuit in the lamp ballast. Other reasons for utilizing feedback circuits in lamp ballasts are: (1) trying to maintain constant current to the lamps if voltage input to the ballast varies since some high frequency ballast power circuits may amplify any utility voltage variation and therefore a feedback circuit is used to correct for the variation; (2) fluorescent lamps require more voltage to start the electrical discharge in the lamp and then a feedback system may be necessary to make corrections when the lamp is lit; (3) some AC-DC power conversion systems in the ballasts require feedback circuits to provide the desired voltage and current to light the lamps; and (4) to assure that the electrical ballast output does not vary even when the electric utility voltage is varying. In contrast, the present invention, as will be discussed later, does not utilize any sort of feedback.
In addition, the ballast of U.S. Pat. No. 5,233,273 (Waki et al.) pertains to the starting and control of a single discharge lamp of the type used in commercial buildings and outdoor lighting whereas the present invention is designed for a plurality of fluorescent lamps. In particular, the ballast circuit of the '273 patent applies only to a metal halide type of discharge lamp that is physically and electrically different in construction (e.g., there are no filaments) than fluorescent lamps. Furthermore, discharge lamps require much higher voltages to "turn on" (e.g., thousands of volts to create the initial breakdown) as opposed to fluorescent lamps which require a much smaller voltage since fluorescent lamps utilize a continuously heated filament at both ends of the lamp. Metal halide discharge lamps require a first formation of an initial breakdown of the gases in the lamp, then a "hot spot" forms which later transitions to an arc between the electrodes and it is the arc discharge in the metal halide lamp that produces the light; in contrast, the fluorescent lamps used in the present invention do not operate at all like metal halide lamps. In addition, connecting the electronic ballast of the '273 patent to one or more fluorescent lamps would result in, at best, a very short life of the fluorescent lamp. In particular, the sockets installed in the fluorescent lamp fixtures would not be able to withstand the high voltage which is necessary to light a metal halide lamp. The '273 patent ballast requires a pulse voltage and, thus, a pulse transformer, neither of which is necessary or desirable for a fluorescent lamp. Furthermore, the '273 patent ballast uses a series resonant circuit whereas the present invention does not utilize such a circuit. The '273 patent must use a lamp current detector whereas the present invention does not utilize a lamp current detector. Furthermore, the '273 patent ballast requires the use of a feedback signal to control its oscillator in order for the ballast to work properly whereas the present invention ballast circuit requires no lamp feedback nor any other type of feedback to control the high frequency signal or the lamp intensity. As a result, the present invention is less complex, requires fewer parts and provides a lower cost to manufacture.
U.S. Pat. No. 4,698,554 (Stupp et al.) discloses a ballast that includes no isolation transformer and, as a result, power frequency voltage and DC power are available at the fluorescent tube sockets, thereby creating a shock hazard. In addition, the '554 patent ballast uses one filament transformer for powering the lamp filaments, whereas the fluorescent lamp filaments in the present invention are powered from the output transformer which also isolates the fluorescent lamp sockets from utility power for safety. The '554 patent ballast also controls lamp current and light intensity by varying frequency using a feedback system, resulting in more components than required by the present invention.
The '020 patent ballast also contains a feedback signal processor and power factor controller, as well as other expensive components. In contrast, the present invention does not utilize any pulse width modulation, no power controller and no feedback signal processor nor power factor controller.
The '915 patent ballast uses a controlled chopper circuit and an inverter circuit. The chopper circuit controls the DC voltage to the inverter which, in turn, provides the high frequency current to the fluorescent lamp. After the lamp is lighted, then the chopper control provides the desired amount of DC current to the inverter which, in turn, provides the proper higher frequency current to the fluorescent light. In contrast, none of this control circuitry is utilized in the present invention. Furthermore, the high frequency voltage is terminated when the lamp tube is removed from the '915 patent ballast. In contrast, the high frequency voltage is still present when a lamp tube is removed from the present invention but the power frequency from the utility is isolated; thus, in the present invention, the power is not shut off when one lamp tube is removed, thereby allowing one ballast to continue to power the remaining lamps if one is removed while still providing the shock isolation.
The '090 patent discloses an electronic ballast system including one or more gas discharge lamps without standard filaments; the filaments are replaced with unconnected single electrodes. This patent shows the use of isolation transformers between the ballast circuit and the lamps; however, like the other references, this ballast also uses lamp circuit feedback.
Other lamp ballast circuits that utilize feedback are:
U.S. Pat. No. 4,538,095 (Nilssen) discloses an electronic ballast circuit that utilizes a feedback circuit, namely circuit A shown in FIG. 1 of that patent.
U.S. Pat. No. 4,675,576 (Nilssen) discloses an electronic ballast that does not utilize an isolation transformer but does utilize a feedback circuit DA for disabling the high frequency power; however, it is the low frequency (50/60 Hz) utility power that is fatal and which is not disabled by that circuit.
U.S. Pat. No. 5,173,643 (Sullivan et al.) discloses a dimmable ballast that utilizes five feedback circuits: a feedback circuit that makes it easier to set the light intensity when the intensity is low; a second feedback circuit is used to control lamp current; a third feedback circuit is used for shutting off the dimming circuit when the voltage is too high; a fourth feedback circuit is used for shutting down the dimming circuit if the ground fault is too high; and a fifth feedback circuit is used for detecting out-of-phase current which flows through the lamps so that sufficient in-phase current is always available to the lamp. In contrast, the present invention does not require any of these.
U.S. Pat. No. 5,192,896 (Qin) discloses an electronic ballast that effectively implements feedback by sensing over-voltage to prevent damage to the ballast or lamps. In contrast, the present invention does not utilize any type of over-voltage sensing.
U.S. Pat. No. 5,539,281 (Shackle et al.) discloses a dimmable ballast that utilizes boost or buck voltage to control lamp power, none of which is used by the present invention.
U.S. Pat. No. 5,825,137 (Titus) discloses non-dimmable and dimmable electronic ballasts for controlling at least two or more fluorescent lamps without any feedback circuitry.
While some prior art electronic ballasts may be generally suitable for their intended purposes, they nevertheless leave something to be desired from one or more of the following standpoints: safety, reliability, ability to provide power to multiple lamps, simplicity of construction, and cost. Many of these prior art electronic ballasts involve complex circuitry due to the fact that they use feedback. Furthermore, there is a need for dimmable ballasts which protect against points of resonance over the range of input frequencies, which cause "blooming" (increased brightness, then dimming), at specific resonant frequencies.